Cloned suppressor T lymphocytes that induce donor tolerance to host tissues in vivo would be useful in studies on transplantation immunology and immunoregulation. This project integrates in vivo and in vitro techniques into a systematic study of the role of cloned immunoregulatory suppressor T cells in transplantation tolerance. Suppressor-like T cells will be used to define the immunogenetic requirements for manifestation of suppression in vivo and in vitro and to gain an understanding of the immunologic and metabolic mechanisms involved in suppression. Prototype clones (SAC-A9) have been isolated from bacteria-free allogeneic bone marrow chimeras (ABMC) with suppressed antihost reactivity. These clones will be characterized as to expression of cell surface antigens, lymphokine receptors and production of lymphokines. We hypothesize that SAC-A9 or similar cells may be involved in maintaining donor-host tolerance in vivo. The migration pattern and functional life-span of clones in vivo in the presence and absence of specific antigen stimulation and/or exogenous IL-2 will be examined. Limiting dilution proliferation and cytotoxicity assays, MLC/CML assays, lymphokine-inhibition assays in vitro and leukemia rejection in vivo will be used to evaluate suppression of alloreactivity in normal mice and ABMC given cloned suppressor cells. The ability of cloned suppressor cells to prevent or treat graft-versus-host disease in vivo will be studied.